In a digital communications system, data is represented by a number of discrete signal levels, for example, where the system allows only two discrete signal levels, the digital system is known as a binary system. In a binary system, the discrete signal levels are named bits and are given a LOGIC 1 or a LOGIC 0 value. In an optical communications system employing a binary system, a data stream comprising on (LOGIC 1) and off (LOGIC 0) bits can be transmitted by a transmitter in the form of an optical signal across an optical network of fibres and amplifiers to a receiver.
In order to interpret the transmitted data stream, the optical signal can be detected at the receiver by a photodetector to generate an electrical signal. On being communicated to a comparator, for example, a limiting amplifier, the electrical signal is compared to one or more threshold levels. A threshold level is a signal level, for example a voltage, defining a boundary between the upper and lower signal levels (the amplitude) of the electrical signal. An electrical signal having a signal level above the threshold is given a LOGIC 1 value, and an electrical signal having a signal level below the threshold is given a LOGIC 0 value. A threshold level can be expressed as a percentage of the amplitude of the electrical signal by:
                              Threshold          ⁢                                          ⁢          %                =                              (                                                                                V                    TH                                    -                                      V                                          D                      ⁢                                                                                          ⁢                      C                                                                                        V                  AMP                                            +                              1                2                                      )                    ×          100                                    (        1        )            
Where, VTH is the threshold level expressed as a voltage; VDC is a DC level of the electrical signal representing an average fixed reference value of the amplitude of the electrical signal; and VAMP is the amplitude of the electrical signal expressed as a voltage being the difference between the upper and lower voltage levels of the electrical signal.
A measure of the quality of a communications link between a receiver and a transmitter is known as a Bit Error Rate (BER). The BER is a measure of the fraction of transmitted bits incorrectly interpreted at the receiver. For adequate performance of the communications link, a Network Equipment Manufacturer (NEM) typically requires a BER below one error bit in 1010 bits. One significant factor affecting the BER of an optical communications system is the signal to noise ratio of the optical signal. Noise can be added to the optical signal as a result of factors such as amplification of the optical signal, timing jitter and the bandwidth limitations of components employed in the communications network which process the data stream; As is known in the art, in terms of a data stream viewed as an eye diagram on a digital communications analyser, noise can cause the eye to close and optical amplifiers can add noise that cause the eye to become asymmetrical. As the eye closes and becomes asymmetrical, the margin or distance between the two logic levels becomes less, and therefore errors in determining whether a received signal represents a LOGIC 1 or a LOGIC 0 bit increases, thereby increasing the BER.
For example, for a 101010 data stream having a high signal to noise ratio and therefore a nearly ideal eye diagram, the optimum threshold level would correspond to substantially half-way between the maximum and minimum signal levels of the electrical signal generated by the receiver in response to the received data stream. Such a threshold level is known as a 50% threshold level in the art and can be derived from equation (1) above. However, should the 101010 data stream undergo several stages of amplification prior to being received at the receiver, noise may be introduced into the data stream causing the probability statistics of the data stream to change and the eye diagram to become asymmetrical. For such a data stream, the optimum threshold level could be at a signal level corresponding to a 40% threshold level. In order to maintain the BER at an acceptable level, it is known in the art that the optimum threshold level must be determined accurately and maintained to within a few per cent of its optimum value.
It is common for the threshold level to be controlled by a microprocessor, for example, it is known to interface a Digital-to-Analogue Converter (DAC) to a limiting amplifier. A suitable DAC may comprise 10 bits to provide 1024 discrete output levels and is typically programmed to operate over the full amplitude range of the electrical signal. For example, if the DAC is set for an amplitude of 400 mV, each discrete level of the 1024 discrete levels is set to a step resolution of 0.39 mV and is therefore capable of adjusting the threshold level to a resolution of 0.1% over the full amplitude range of the electrical signal. However, should the amplitude of the electrical signal fall to, for example, 5 mV as a result of a fall in the power of the received optical signal, each discrete level at a step resolution of 0.39 mV would only be capable of adjusting the threshold level to a resolution of 8% of the 5 mV amplitude of the electrical signal. Such a coarse resolution can result in poor control of the threshold value and therefore an increase in the BER as discussed above.
As such, NEMs must have the capacity to select an optimum threshold level at any value from the full amplitude range of an electrical signal. Also, a NEM must have the capability, at a suitable resolution, to maintain control of the threshold level to within a few per cent of its optimum value in an event of a change in power of a received optical signal and hence a change in amplitude of the generated electrical signal.
In an attempt to meet the above requirements, it is known to employ an Automatic Gain Control (AGC) amplifier prior to the limiting amplifier in order to provide a fixed value for the amplitude of the generated electrical signal independently of a change in the power of the received optical signal. In the above case, the AGC output would be fixed to 400 mV thereby ensuring that the amplitude range of the electrical signal communicated to limiting amplifier is fixed at 400 mV despite a fall in input signal power in order to ensure that the DAC is capable of adjusting the threshold level to a resolution of 0.1% over all input optical signal powers. A disadvantage of employing an AGC amplifier is that the extra amplifier increases manufacturing costs and power consumption in addition to using valuable circuit real estate.
Alternatively, an adaptive decision threshold setting circuit such as one disclosed in GB 2 293 931 can be employed. In GB 2 293 931, an input digital signal developed by a signal source is time averaged and mirrored by a current mirror to generate one or more decision thresholds. The thresholds may be a set or variable fraction of the average current and track any variation in the average current obviating a need for an AGC amplifier. A disadvantage of GB 2 293 931 is that in order to keep noise in the low level digital signal developed by the signal source to a minimum, it is necessary to redesign existing integrated circuits to realise the design on the first amplifier chip, thereby increasing costs and development times.